1. Field of the Invention
The present invention relates to a quantity of flow meter, in the case of which a flowing medium is directed past two current carrying conductors which have a temperature-dependent resistance, in the case of which one electrical conductor is heated by the current and serves as a measuring resistor and the second conductor serves as a comparator resistor and is not heated by the current. More specifically, the invention relates to a quantity of flow meter of such type in which further resistors are provided which, together with the measuring resistor and the comparator resistor, form a resistance measuring bridge, and the provision of devices for electronically controlling bridge current in dependence upon the voltage at a bridge tap, the devices regulating the voltage at the bridge tap to zero, the change of bridge current being evaluated as a measurement for the quantity of the flowing medium.
2. Description of the Prior Art
A flow meter of the type set forth above is known from "Bosch Technische Berichte 5 (1975) 1" in which the comparator resistor and the measuring resistor are formed by clamped thin wires. For the comparator resistor, a significantly thinner wire is necessary than for the measuring resistor, so that a comparator resistor can be produced with such a high resistance value that it does not heat to an interfering degree by means of the current flowing therethrough, in contrast to the measuring resistor. The sizes of the wires are set at lower limits because of the necessary mechanical strength of the wires. Therefore, as a result of the small dimensions of the flow meter which are required, the resistance cannot be selected to be so high that a sufficient accuracy of measurement is attained. The dimensions required, for example in motor vehicle construction, of in any case but few centimeters, produce such low resistance values that the bridge can be operated only with voltages in the order of magnitude of 1 V. In the case of such small voltages, for example, in flowing liquids, galvanic voltages between different parts of the circuit, which are to be avoided, come into play. Such voltages cause errors in the measured result in a manner which is not controllable. In flowing gases, for example air, as a result of the low resistance values, in the case of a bridge voltage of 1 V, at the measuring resistance, excess temperatures of about 200 K. arise, which overload the measuring resistance for a long time and are not permissible for many flowing media. In this respect, an excess temperature up to 40 K. is not dangerous, and such a temperature can only be set in the case of resistance wires with bridge voltages significantly below 1 V, whereby the bridges operate inaccurately.